Matrixes and decoders for quadruphonic records

ABSTRACT

Method and apparatus for recording four separate channels of information on a medium having only two independent tracks, such as a phonograph record, and method and apparatus for reproducing such information and presenting it on four loudspeakers to give the illusion of sound coming from four separate sources. The signals recorded in the manner described in this application are also reproducible on conventional stereophonic playback systems, distributing the four separate channels to the two loudspeakers in a manner to give a balanced and symmetrical reproduction. Two embodiments of encoding apparatus for combining the four input signals preparatory to recording on the two-track medium, and two decoders, one for each of the encoding systems, are described.

United States Patent 91 Bauer m 3,745,252 July 10, 1973 MATRIXES AND DECODERS FOR QUADRUPHONIC RECORDS Benjamin B. Bauer, Stamford, Conn.

Inventor:

[73] Columbia Broadcasting System, Inc.,

New York, NY.

Filed: Feb. 3, 1971 Appl. No.: 112,168

Assignee:

US. Cl 179/1 GQ, 179/1004 ST Int. Cl. H04r 5/00 Field of Search 179/15 ET, 1 G, 1 GP,

179/1004 ST, 100.1 TD

References Cited UNITED STATES PATENTS 1/1972 Scheiber 179/15 BT OTHER PUBLICATIONS Four Channels and Compatibility by Scheiber Audio Engineering Society Preprint, Oct. 12, 1970.

A New Quadraphonic System by Hafler Audio Magazine, July 1970.

Primary Examiner Kathleen H. Claffy Assistant Examiner-Thomas DAmico Attorney- Spencer E. Olson and Martin M. Novack 57 ABSTRACT I Method and apparatus for recording four separate channels of information on a medium having only two independent tracks, such as a phonograph record, and method and apparatus for reproducing such information and presenting it on four loudspeakers to give the illusion of sound coming from four separate sources. The signals recorded in the manner described in this application'are also reproducible on conventional stereophonic playback systems, distributing the four separate channels to the two loudspeakers in a manner to give a balanced and symmetrical reproduction. Two embodiments of encoding apparatus for combining the four input signals preparatory to recording on the twotrack medium, and two decoders, one for each 'of the encoding systems, are described.

10 Claims, 13 Drawing Figures Puma July 10, 1913 3,145,252

5 Shoots-Sheet 1 INVENTOR. BENJAMIN B. BAUER BY Anew;

A7 TORNE Y Pate ted July 10, 1973 4 3,745,252

5 Sheets-Sheet I;

BENJAMIN B. BAUER ATTORNEY Patented July 10, 1973 3,745,252

5 Sheets-Sheet b 1N VEN'I'OR. BENJAMIN B. BAUER ATTORNEY BACKGROUND OF THE INVENTION This invention relates to apparatus for recording and reproducing four separate channels of information on a medium having only two independent tracks, and more particularly to improved apparatus for recording such information and for reproducing the information and presenting it on four loudspeakers to give the listener the illusion of sound coming from a corresponding number of separate sources. In a system of this kind described in applicants co-pending US. application Ser. No. 44,224 filed June 8, 1970, entitled Quadruphonic Recording and Reproducing System and assigned to the assignee of the present invention, in which a stereophonic record, which may be in the form of disc or tape, etc., is used as the two track medium, there are recorded on the left and right channels signals to be presented on the left front and right front loudspeakers, respectively, together with the signals on both channels identified with left back and right back loudspeakers at 90 out of phase relative to each other, with the left back signals leading the left front channel signal on the left track and the right back signals leading in the right front channel signal on the right track. Also described in aforementioned application is a decoding system which accepts the two outputs from the disc record, one from each track, and by appropriate electronic manipulation separates them into a simulation of four independent channels, for presentation on four separate loudspeakers, each carrying predominantly the information contained in the orginally recorded sound channels with attenuated information from other channels.

There being considerable current interest in multiple-channel recording and reproduction systems of this general type, which are termed stereo quadruphonic" systems, other techniques have been suggested for matrixing the four channels of information preparatory to recording on the two-track medium, and for decoding the information to give the desired illusion of four channels of information upon reproduction. In one such system of which applicant is aware, described in an Audio Engineering Society Preprint entitled Four Channels and Compatibility by Peter Scheiber, Oct. 12, 1970, the encoding matrix is as diagrammatically illustrated in FIG. 1 of the drawings, and phasor diagrams useful in explaining the operation of the decoder are presented in FIGS. 2 and 3. The encoding matrix of FIG. l is operative to encode four separate channels of program information designated L, (left front), L, (left back), R (right back) and R, (right front) into two new channels L and R The four signals are respectively,

applied to input terminals 2, 4, 6, 8, each of which is connected to both of a pair of summing devices and 12 as shown. The summing device 10, which may be a matrix of operational amplifiers and resistors, is operative to add 0.924 of signal L 0.924 of signal L -0.383 of signal R and 0.383 of signal R,, these factors being indicated by the numbers adjacent the point of connection of the respective terminals to summing circuit 10. The summing device 12, of similar construction, sums 0.383 of signal L,, 0.383 of signal L 0.924 of signal R and 0.924 of R, lt is to be noted that 0.383 is the sine of 22 W while 0.924 is the cosine of 22 AP.

The resulting composite signals L and R, from summing circuits l0 and 12, respectively, may be applied to the transducers of a recorder, which may be either a two-track magnetic tape recorder or a stererophonic disc cutter, etc. The form of the composite signals is shown in phasor form in FIG. 2, FIG. 2A representing the signal L applied to the left recording track, and FIG. 2B representing the total signal R applied to the right recording track. It will be noticed that the phasor corresponding to signal L is made up of two equal signals L, and L,,, each 0.924 long and of the same phase, and two signals 0.383R and 0.383R,, also equal in length but directed oppositely. Likewise, the signal R is made up of two larger signals, 0.924R, and 0.924R,,, of the same phase, and two smaller signals 0.383L and 0.38L, of opposite phase. It should be noticed further that while, in general, signals L,, L R, and R, are complex program signals which cannot be represented by phasors, if we consider their relationship on a signal frequency basis then the use of the phasor representation is justified and it serves to better visualize the function of the apparatus.

A recorded program comprising the signals L and R, can be reproduced on a conventional stereophonic player, or it can be de-matrixed in a special resistive dematrixing network, the details of which are not of concern to the discussion to follow. Suffice it to say that the de-matrixing network produces four signals each of which is composed of a predominant signal L,, L,,, R and R,, respectively, together with two additional signals of the same series at a 3dB lower level, diluting the predominant signal.

From the discussion to follow it will be seen that the above-described matrixing technique produces results which seriously detract from the realism of four channel operation. Consider, for example, the operation of the network when equal signals, L, and R,, representing a center-front signal are applied to terminals 2 and 4. It will be observed that in this situation the signal L becomes elongated, forming a new signal, designated A, since 0.924 0.383 =,l.307. The signal R also becomes elongated, for the same reason, forming a new signal, designated B, which is also 1.307 long. However, if one wishes to record a center-back signal by applying equal signals L and R, to terminals 4 and 6, respectively, then the signal L becomes foreshortened by a factor 0.924 0.383 =0.S4l, forming a new signal C, and the composite signal R, is likewise foreshortened, forming a new signal D, which is also 0.541 long. It isseen, therefore, that the efficiency of recording a signal applied to the back terminals differs from the efficiency of recording a signal applied to the front terminals; this has been considered to be a serious deficiency of this matrix.

In an attempt to improve some of the characteristics of the just-described matrix, it is suggested in the aforementioned Audio Engineering Society Preprint that a relative phase shift of be introduced between the signals L and R, as by inserting all-pass shift networks 14 and 16 between the output terminals to produce a new set of signals L and R, which are then applied to recorder. While the inclusion of this relative 90 phase shift improves the performance from some points of view, it causes significant deterioration in other respects. For example, upon application to the matrix of equal inputs L, and R, (as one would do to obtain a center-front" signal), the two new phasors which are 3 obtained, namely, A and B in FIGS. 3A and 38, re-

spectively, are at 90 with respect to each other, thus SUMMARY OF THE INVENTION It is the principal object of the present invention to provide an improved method and apparatus for matrixing four signals which results in a vastly superior arrangement of the matrixed signals and which is free of the aforementioned defects of the prior art matrix illustrated in FIG. 1.

Briefly, this object is obtained with a matrix which places the four original signals in appropriate phase relationship by the use of suitable all-pass networks prior to summation into the two final signals L and R More specifically, to achieve this object with a minimum number of all-pass networks, the L, and L signals are twice summed, 0.924 of each being added together in a first summing network, and 0.383 of L, and O.383 of L, being added in a second summing network. Similarly, 0.924 of each of signals R and R, are added in a third summing network and 0.383 of signal R, and O.383 of signal R, are summed in a fourth summing network. The outputs of the first and third summing circuits are summed in a fifth summing circuit after introducing a 90 phase-shift in the output of the first sum signal relative to that of the third sum signal, and the outputs of the two remaining summing circuits are similarly combined in a sixth summing network, the outputs of the fourth summing network being subjected to a 90 phase-shift with respect to the output from the second summing network. The outputs of the fifth and sixth summing networks are the composite signals L and R respectively. These composite signals may be applied to a stereophonic loudspeaker system directly, or recorded on a tape recorder or a stereophonic disc recorder for subsequent replay on a two-channel stereophonic system. By combining the signals in this manner, the front and back signals have symmetry and the phase relationship of the center signal is improved.

In another embodiment of the invention,in which essentially the same matrix is utilized, the front-to-back symmetry is altered (which in some applications is desirable by introducing a different phase-shift between the ultimately summed signals than was used in the justdescribed embodiment).

In both embodiments there is an enhanced measure of performance during reproduction on a stereophonic system, and at the same time provides improved performance on a four-channel de-matrixed quadruphonic system.

BRIEF DESCRIPTION OF THE DRAWINGS An understanding of the foregoing and additioal objects and aspects of the invention may be gained from consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic diagram of the prior art system to which reference has already been made;

FIGS. 2 (A and B) and 3 (A and B) are phasor diagrams to which reference has been made in the discussion of the system of FIG. 1;

FIG. 4 is a schematic diagram of an encoding matrix embodying the present invention;

FIG. 5 (A and B) are phasor diagrams useful in explaining the operation of the circuit of FIG. 4;

FIG. 6 is a schematic diagram of a system for decoding signals recorded with the encoder of FIG. 4;

FIG. 7 is a schematic diagram of a variation of the encoder of FIG. 4;

FIG. 8 (A and B) are phasor diagrams useful in explaining the operation of the encoder of FIG. 7; and

FIG. 9 is a schematic diagram of a portion of a reproducing system for decoding signals recorded with the encoder of FIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred form of encoder embodying the present invention is illustrated in FIG. 4 and is similar in some respects to the encoder described in applicant's aforementioned co-pending application Ser. No. 44,224, differing therefrom in the manner in which the signals are summed and in the phase-shift constant used. The four separate channels of program information, again designated L,, L,,, R, and R for left front, left back, right front, and right back, respectively, are applied to input terminals 18, 20, 22 and 24 of the encoder. Terminals 18 and 20 are both connected to a pair of summing devices 26 and 28, and input terminals 22 and 24 are both connected to another pair of summing devices 30 and 32. Summing devices 26 and 32 are of similar construction, both being operative to add the decimal fraction 0.924 of each of the two signals applied thereto. ,Summing devices 28 and 30 are alike but differ from summing devices 26 and 32 in that 0.383 of one of the signals applied to each is added to O.383 of the other applied signal. More specifically, in the case of summing device 28, 0.383 of the signal L, is added to O.383 of the signal L,,, and in device 30, 0.383 of the signal-R, is added to O.383 of the signal R,.

The outputs of the four summing devices 26-32 are applied to all-pass phase-shift networks 34, 36, 38 and 40, respectively; The networks 34 and 40 each introduce a phase-shift of (III and the networks 36 and 38each introduce a phase-shift (all 0) into their respective circuits, the reference angle in being arbitrarily chosen, the only requirement being that this reference angle is substantially the same in any one encoder or decoder. It should be noted that although a convention that these phase-shift angles are lagging has been observed, leading phase-shifts could also be employed, as long as the same convention is observed in any one encoder or decoder. The respective phase-shift networks introduce the differential phase angles shown; that is, networks 34 and 40 introduce a phase-shift of 90 in the signals from summing devices 26 and 32 relative to the signals from summing devices 28 and 30. The outputs of phase-shift networks 34 and 38 are added in equal proportions in an additional summing device 42, and the output signals from phase-shift networks 36 and 40 are similarly added in summing device 44. Summing devices 42 and 44 produce at output terminals 46 and 48, respectively, the final composite output signals oppositely directed, but which also are at 90 with respect to the signals L, and L,,. Likewise, the signal R, is composed of two relatively large signals, 0.924R, and 0.924R,,, which are in phase, and two smaller signals, 0.383L, and 0.383L which are directed in opposite phase, and at the same time are at 90 to the larger signals R, and R This 90 relationship between the larger and smaller signals gives the present decoder a significant advantage in that if now a front-directed signal consisting of equal measures. of signals R, and L, is

applied to the matrix, the resulting signals will be as denoted by the phasors A and B, shown in dashed lines in FIGS. 5A and 5B, respectively, which, it will be noted, are at 45 relative to each other. This does not meet fully the earlier stated desirable condition that these phasors be in phase, but the results are much improved from that attainable in the prior art device in which the corresponding phasors are at 90 to each other (FIG. 3). At thesame time, if one applies a back-directed signal, by applying equal measures of L,, and R to terminals 20 and 22, respectively, the resulting total signals are shown by the dashed-line arrows C and D in FIGS. 5A and SB, respectively. It will be observed that phasors C and D are of the. same magnitude as the A and B phasors obtained when only a front-center signal is applied. Thus, the encoder of FIG. 4 has symmetry front-to-back and provides an improvement with respect to the in-phase tendency of the center signal.

One important consideration in connection with the form of the invention illustrated in FIG. 4 is that the phasor pairs A and B are of equal magnitude, but of opposite relative phase positions compared to the phasors C and D which result from the application of backcenter? signals. This allows the advantageous possibility during decoding to specifically distinguish between front-center and back-center signals which other decoding schemes do not allow. It should be noted that the signs of the summation network 30 can be reversed without significant deterioration of the operation of the encoder, and such reversal would result in the signals L and R, being fully in phase for the application of either the front-center or the back-center signals. However, such an encoder would not allow distinguishment in the decoding process between the frontcenter and the back-center signals.

It is significant to note that the step of summing the signals L,, L,,, R and R, with the summing networks 26,

28, 30 and 32 prior to phase-shifting with the networks 34, 36, 38 and 40 results in an economy of circuitry. Instead of employing these first-mentioned summing operations, one could provide, for example, eight separate phase-shifting networks followed by summation networks 42 and 44 at which the four appropriate phasors are summed in the required proportion. By using the four initial summing operations, only four phaseshift networks are required in the encoder in place of eight.

Referring now to FIG. 6, there is illustrated one form of apparatus for decoding the stereophonic signal encoded with the encoder of FIG. 4 into four signals which carry predominantly the original information (albeit diluted with the information from two adjacent channels) for presentation over a four loud-speaker system. The two signals L and R, are derived from the record medium by a suitable transducer (e.g., a conventional stereophonic pickup in the case of a disc record) and are applied to terminals 50 and 52, respectively, of the decoder apparatus. In order to position these signals properly for de-matrixing, the signals L and R, are applied to differential phase-shift networks 54 and 56, respectively, which differentially alter the position of the phasors L and R, by 90, in accordance with the teachings of applicants aforementioned copending application Ser. No. 44,224. The relative orientation of the two signals after phase-shift are as depicted'in the phasor diagrams associated with the output terminals of phase-shift networks 54 and 56. These signals are applied to each of four summing devices 58, 60, 62 and 64 in the proportion indicated in the blocks representing the summing devices. That is, in summing device 58, 0.924 of the L signal is added to 0.383 of the R, signal, in summing device 60, 0.924 of L, is added to 0.383 of R in summing device 62, 0.383 of L, is added to 0.924 of R and in summing device 64, 0.383 of L, is added to 0.924 of R Without going into the geometry, which is believed will be selfevident, summation of the signals'in these proportions produces four new signals L,', L,,, R, and R, at the output terminals 66, 68, and 72 of summing devices 58, 60, 62 and 64, respectively, the phasor diagrams of which are depicted adjacent their respective terminals. It will be noticed that these phasors have respective predominant signals L,, L,,, R, and R, in combination with flanking signals from adjacent terminals. This is considered by some to be a preferred way of forming a quadruphonic signal. By contrast, if one were to reverse the signs of summation, say in the summing network 30, as herein before described, this would invert the small phasors in one 'of the signals portrayed in FIGS. 5A or 5B for example, causing the position of smallphasors 0.383R and 0.383R, to be reversed. It will be found that under this circumstance upon dematrixing, each of the diluted signals would have a predominant signal in combination. with two opposite-end signals, which according to some listeners produces less desirable results.

It is to be noted that while the phasors in FIG. 5 are tagged with signsand phase angle positions, this practice has not been followed in FIG. 6 in the interest of avoiding confusion and unnecessary repetition. In FIG. 6, rather, sign and phase are are shown simply by the relative position of a phasor in the phasor diagram.

After de-matrixing, it may be desirable to place the principal phasors L,, L,,, R, and R, appearing at terminals 66, 68, 70 and 72 in some other phase orientation relative to each other; for example, it may be desired to have them all in phase. This may be accomplished by applying the signals to respective all-pass phase-shift networks 82, 84, 86 and 88, the latter two of which introduce a relative phase-shift of 90 relative to the other two. The signals delivered by the phase-shift networks may then be amplified by respective gain control amplifiers 90, 92, 94 and 96, the gain of each of which may be controlled by applying a 'control'signal to its '7 control electrode and applied to respective loudspeake'rs 98, 99, 100 and 101.

Should it be desired to enhance the quadruphonic illusion of the reproduced signals, the gains of amplifiers 90, 92, 94 and 96 may be controlled by a control and switching logic network 102 in response to signals derived from the output terminals of phase-shifters 54 and S6 in the manner described in co-pending application Ser. No. 44,196 filed June 8, 1970 by applicant and Daniel W. Gravereaux, and assigned to the as signee of the present application. Briefly, the control and switching logic 102 processes the signals appearing at the output terminals of phase-shifters 54 and 56 through an automatic gain control circuit which maintains them at a constant predetermined level. They are then de-matrixed in a circuit similar to that formed by summing circuits 58, 60, 62 and 64, the resulting four signals are summed with an appropriate time-constant network, and the sum signal fed back to the automatic gain control circuit so as to maintain the sum substantially constant. The individual signals are combined through linear additions and subtractions to produce control signals at the output lines 104, 106, 108 and 110 which are respectively connected to the gain control terminals of amplifiers 90, 92, 94 and 96. The system is designed to apply control signals to the gain control amplifiers to increase the gain of the channel containing the instantaneously dominant signal and to reduce the gain of the other channels to give a substantially perfect illustion of four separate independent sources of sound. As the sound diminishes in the channel first identified and another sound appears on a different channel, the logic circuit functions to rapidly attenuate the gain in the first channel and to increase the gain in a different channel. In this manner, it is possible to produce a high level of simulation of four discrete channels.

Not only are these four channels produced in a highly realistic manner, but because of the symmetry of the phasors, it is possible to 37 pan a signal between any two adjacent pairs of input terminals, i.e., L, L,,; L, R and R R,; and R,-- L,, without significant change of level during the panning operation. Furthermore, if the record is played on a conventional two-track stereophonic system, the signals appear to be relatively crisp and well defined.

Referring now to FIG. 7, there is shown another embodiment of the invention by which the front-back symmetry of the signals is changed, which for some applications might be preferable. The general configuration of the encoder resembles the system of FIG. 4, but aswill be seen from the discussion to follow the signals are combined in a different way. The four original input signals L,, L,,, R, and R, at the terminals 112, 114, 116 and 1 18, respectively, are applied to four summing devices 120, 122, 124 and 126 in the manner and in the proportions indicated by the decimal fractions appearing on the summing devices. More specifically, 0.383 of each of signals L, andL, are added in summing device 122 and 0.924 of each of them are added in summing device 124. Similarly, 0.383 of each of the R, and R, signals are summed in summing device 120, and 0.924 of each of them are summed in summing device 126. The outputs of summing devices 120, 122, 124 and 126 are connected to respective all-pass phaseshift networks 128, 130, 132. and 134 which produce relative phase-shifts to the signals transmitted thereby of 0, 45, and The outputs of phase-shift networks 128 and 132 are added in equal proportions in summing circuit 136, and the outputs of phase-shift networks 130 and 134 are similarly added in equal proportions in summing device 138. The composite signals L and R appearing at the output terminals and 142 of summing devices 136 and 138, respectively, may be recorded on two-track magnetic tape or on a stereophonic disc in conventional manner for subsequent replay over a stereophonic system, or they may be de-matrixed into four signals as hereinafter described.

Phasor diagrams of the composite signals L and-R are displayed in FIGS. 8A and 88, respectively. For convenience, these phasor. diagrams are presented in reference to a geometrical system of coordinates the axes of which are marked 0, 45, 90 and etc., albeit the actual electrical phase will be drawn with respect to an imaginary electrical axis which is labeled 0' axis. Thus, in FIG. 8A, the signal 0.383R which undergoes the minimum phase-shift n11, is shown as being coincident with the 0' axis, while the 0.924L signal, which undergoes .a relative phase lag of +90, is shown lagging behind the phasor 0.383R, by 90, which places it at 22 2% to the geometrical 90 axis. Similarly, in FIG. 8B, the signal 0.383L,, which undergoes the phase-shift Ill +45, is displaced 45 from the 0' axis, while the 0.924R, signal, which undergoes a relative phase lag of +90, is shown lagging behind the phasor 0.383L, by 90, which places it at 22 15 to the geometrical 90 axis.

One important advantage of the matrixor on FIG. 7 will now be demonstrated, Let it be assumed that a center-front signal is applied to the matrix by applying equal L, and R, signals to terminals 112 and 118, re-

spectively. It will be seen from reference to FIG. 8A

that in this case the signal L, is composed of 0.924 parts of L, and 0.383 parts of R, which resolve into the dashed-line phasor A, which it will be noted, lines up precisely with the geometrical 90 axis. At the same time, the total signal R shown in FIG. 8B is comprised of 0.924 parts of R, and 0.383 parts of L, which form the dashed-line phasor B, which has a length of unity and which also lines up precisely with the geometrical 90 axis. Therefore, the two phasors produced at terminals 140 and 142 are equal and in-phase and will, therefore, produce a precise and sharp center signal.

The situation is different, however, when a backcenter signal is applied to the matrix by applying equal in-phase signals L, and R, to terminals 114 and 116. In this case, the output L signal, shown in FIG. 8A, is comprised of 0.924L, and 0.383R,, which is aligned with the 45 geometrical axis. The R, signal, on the other hand, shown in FIG. 88, consists of 0.924 portions of R and 0.383 parts of L,,, which resolve to form the phasor D, also of unity length but aligned with the 135 geometrical axis. lt will be observed that the phasors C and D are at 90 with respect to each other, and as was noted earlier, it is known that this will produce a fuzzy image upon replay over a stereophonic system. Thus, with the encoder of FIG. 7, application of a center-front signals produces a sharp image and application of a center back signal produces a fuzzy image. This turns out to be a highly desirable method of differentiating front from back when the signals are reproduced over a two loudspeaker stereophonic system. In any case, since all the phasors A, B, C and D have the Referring now to FIG. 9, there is illustrated a dematrixor for decoding the signals encoded by the systern of FIG. 7. The composite signals L and R are applied to terminals 144 and 146, respectively, and thence through respective all-pass phase shift networks 148 and 150 which introduce a relative 45 phase shift between the signals. This phase-shift action causes the two signal phasors to be aligned with respect to each other in such a way that they can be de-matrixed. The phasor diagrams of signals L and R which correspond to FIGS. 8A and 88, respectively, are reproduced for convenient reference adjacent terminals 144 and 146, respectively, except that the markings referring to the lag phase angle and Sign have been eliminated for the sake of clarity. After passing through the phase-shift networks, the phasor diagrams of the resulting signals, designated L and R are as shown associated with the output terminals of the respective phaseshift networks. These latter signals are de-matrixed by applying them to four summing networks 152, 154, 156 and 158 in the proportions indicated in the circles representing the summing devices. The operation of the summing devices, which is generally similar to that described in connection with FIG. 6, produces signals at the respective output terminals 160, 162, 164 and 166 of thesumming devices 152-158 which are predominantly L, L,,, R, and R,,'respectively. The phasor diagrams of the four signals are depicted in association with the respective terminals, it being noted that each predominant phasor is combined with the two flanking phasors from adjacent channels, thereby providing the same quadruphonic image action as in the embodiment of FIG. 5.

It will be understood that the output terminals of the de-matrixor network of FIG. 9 may be connected to additional phase-shift networks to place the principal phasors in any desired phase orientation relative to each other in the manner described in the system of FIG. 6. It will be further understood that the justdescribed phase-shift networks (which should be regarded as optional) may be connected to corresponding gain control amplifiers for application to'respective loudspeakers, and arranged to be controlled by a logic and switching network, again as described in connection with FIG. 6 and in the aforementioned co-pending application.

It will be evident that the herein described modification of the encoding or matrixing circuit disclosed in applicant's aforementioned co-pending application produces an enhanced measure of performance when the record medium is reproduced on a stereophonic system, and at the same time provides improved performance on a four-channel de-matrix quadruphonic system. The latter improvement is achieved because the encoding matrixes described herein do not possess directional ambiguity of the kind inherent in the matrixor of FIG. 1; in the prior art circuit of FIG. 1 there is a directional ambiguity which makes it impossible to be sure whether the encoded signals had originated between the front or back pairs of loudspeakers. Examination of the topology of the FIG. 1 matrix suggests that at least two points of directional ambiguity are bound to exist, and as a matter of fact, exhibit a continuous and broad region of ambiguity, extending along LII the two paths from the left back to the right back loudspeaker. In other words, a sound panned between the input terminals corresponding to these loudspeakers will appear to travel from the left back to the left front loudspeakers, then to the right front, and next to the right back loudspeakers. The present matrixes do not possess such an ambiguity.

I claim:

1. Encoding apparatus for matrixing four signals designated L,, L, R, and R, into two composite signals designated I and R for recording or transmission on a two-track medium,- said apparatus comprising:

first, second, third and fourth input terminals to which said L L R, and R, signals are respectively applied,

first and second output terminals, and

signal transfer means connected in circuit between said input terminals and said output terminals to transfer signals from the input terminals to the output terminals, said signal-transfer means including a first summing circuit connected to add a predetermined first portion of the L, signal from the first input terminal to a like first portion of the L,, signal from the second input terminal to produce'a first sum signal, I

a second summing circuit connected to add said predetermined first portion of the R, signal from the fourth input terminal to a like portion of the R signal from the third input terminal to produce a second sum signal,

a third summing circuit connected to combine in phase opposition a second predetermined portion smaller than said' first predetermined portion of the L, signal from the first input terminal and a like smaller portion of the L, signal from the second input terminal to produce a third sum signal,

a fourth summing circuit connected to combine in phase opposition said second predetermined smaller portion than said first predetermined portion of the R, signal from the fourth input terminal and a like smaller portion of the R, signal from the third input terminal to produce a fourth sum signal,

a fifth summing circuit connected to add said first sum signal to said fourth sum signal and'first allpass phase-shifting means operative to cause the first sum signal received by said fifth summing means to be substantially in phase quadrature with the fourth sum signal received by said fifth summing circuit,

a sixth summing circuit connected to' add said second sum signal to said third sum signal and second allpass phase-shifting means operative to cause the second sum signal received by said sixth summing circuit to besubstantially in phase quadrature with the third sum signal received by said sixth summing circuit, and

means connecting the output terminals of said fifth and sixth summing circuits to said first and second output terminals, respectively, thereby to produce at said first output terminal a composite signal L consisting of said predetermined first portion of said L, and L, signals and said second predetermined smaller portion of said R, and R, signals and wherein theR, signal is substantially in quadrature with one of the L, and L, signals and the R signal is substantially in quadrature with the other of the L, and L signals, and to produce at said second output terminal a composite signal R, consisting of said predetermined first portion of said R, and R, signals and said second predetermined smaller portion of the L, and L, signals and wherein the L, signal is substantially in quadrature with one of the R, and R, signals and the L, signal is substantially in quadrature with the other of the R, and R, signals.

2. Apparatus according to claim 1, wherein said predetermined first portion is substantially the decimal fraction 0.924 and said predetermined second smaller portion is substantially the decimal fraction 0.383.

3. Encoding apparatus according to claim 1, wherein for in-phase input signals L,, L,, R, and R, said summing circuits and said phase-shifting means are operative to cause the L, and L, signals in the composite signal L, and the R, and R, signals in the composite signal R, to all be substantially in phase, to cause the R, and R, signals in the composite signal L, to be substantially in phase opposition with one another, and to cause the L, and L, signals in the composite signal R, to be substantially in phase opposition with one another.

4. Encoding'apparatus according to claim 1, wherein the said all-pass phase-shifting means are operative to shift the phase of said first, second, third and fourth sum signals to cause the relative phase angles of said first, second, third and fourth sum signals to be 90, 135, 45 and respectively.

5. Decoding apparatus for reproducing four different signals from two composite signals L, and R, wherein the L, signal contains a predetermined first portion of I signals designated L, and L, and a second predetermined smaller portion than than said first predetermined portion of signals designated R, and R, and wherein the R, signal is substantially in quadrature with one of the L, and L, signals and the R, signal is substantially in quadrature with the other of said L, and L, signals and wherein the R, signal contains said predetermined first portion of the said R, and R, signals and said second predetermined smaller portion of the said L, and L, signals and wherein the L, signal is substantially in quadrature withone of said R, and R, signals and the L, signal is substantially in quadrature with the other of said R, and R, signals, said apparatus comprismg:

signals are respectively applied, said input circuits including all-pass phase-shifting means operative to shift the relative phase of said L, and R, signals by a predetermined angle to position corresponding components thereof either in phase or in phase opposition to permit later addition or subtraction of said corresponding components, and

first, second, third and fourth signal-combining networks, each having first and second input terminals and an output terminal, each connected to receive at its first and second input terminals a signal from said first and second input circuits, respectively,

said first signal-combining network being operative to combine a portion of the signal from said first input circuit equal to said predetermined first portion with the inverse of a portion of the signal from said second input circuit equal to said predetermined second portion and to produce at its output terminal a first output signal containing one of said L, and L, signals as its predominant component,

and

first and second input circuits to which the L, and R,

said second signal-combining network being operative to combine a portion of the signal from said first input circuit equal to said predetermined first portion with a portion of the signal from said second input circuit equal to said predetermined second portion and to produce at its output terminal a second output signal containing the other of said L, and L, signals as its predominant component,

said third signal-combining network being operative to combine a portion of the signal from said first input circuit equal to said predetermined second portion with a portion of the signal from said second input circuit equal to said predetermined first portion and to produce at its output terminal a third output signal containing one of said R, and R, signals as its predominant component, and

said fourth signal-combining network being operative to combine the inverse of a portion of the signal from said first input circuit equal to said predeter' mined second portion with a portion of the signal from said second input circuit equal to said predetermined first portion and to produce at its output terminal a fourth output signal containing the other of said R, and R, signals as its predominant component.

6. Decoding apparatus according to claim 19 wherein said predetermined first portion is substantially the decimal fraction 0.924 and said predetermined second smaller portion is substantially the decimal fraction 0.383.

7. Decoding apparatus according to claim 5, wherein said input circuits are operative to introduce a relative phase shift of substantially between said L, and R, composite signals, and wherein said first, second, third and fourth output signals respectively contain the L,, L,, R, and R, signals as its predominant component.

8. Decoding apparatus according to claim 5, wherein said input circuits are operative to introduce a relative phase shift of substantially 45 between the L, and R, composite signals, and wherein said first, second, third and fourth output signals respectively contain the L,, L,, R, and R, signals as its predominant component.

9. Encoding apparatus for matrixing four input signals designated L,, L,, R, and R, into two composite signals, said apparatus comprising:

first, second, third and fourth input terminals to which said L,, L,, R, and R, are respectively applied, first and second output terminals, circuit means including means for shifting the phase of said R, input signal at said fourth input terminal by a reference phase-shift angle and for shifting the phase'of said L, input signal atsaid second input terminal by said reference phase-shift angle plus 90 and operative to couple first and second predetermined portions, respectively, of said phase shifted R, and L, signals to said first output terminal,

circuit means including means for shifting the phase of said R, input signal at said third input terminal by said reference phase-shift angle and for shifting the phase of said L, input signal at said first input terminal by said reference phase-shift angle plus 90 and operative to couple said first and second predetermined portions, respectively, of said phase-shifted R, and L, signals to said first input terminal, I

circuit means including means for shifting the phase of said L input signal at said second input terminal by said reference phase-shift angle and for shifting the phase of said R, input signal at said fourth input terminal by said reference phase-shift angle plus 90 and operative to couple said first and second predetermined portions, respectively, of said said reference phase-shift angle and for shifting the minal by said reference phase-shift angle plus 90 and operative to couple said first and second predetermined portions, respectively, of said phaseshifted L, and R,, signals to said second output terminal.

phase-shifted L,, and R, signals to said second out- 10. Encoding apparatus in accordance with claim 9,

put terminal, and wherein said first and second predetermined portions circuit means including means for shifting the phase are respectively the decimal fractions 0.383 and 0.924.

of said L,input signal at said first input terminal by 

1. Encoding apparatus for matrixing four signals designated Lf, Lb, Rb and Rf into two composite signals designated LT and RT for recording or transmission on a two-track medium, said apparatus comprising: first, second, third and fourth input terminals to which said Lf, Lb, Rb and Rf signals are respectively applied, first and second output terminals, and signal transfer means connected in circuit between said input terminals and said output terminals to transfer signals from the input terminals to the output terminals, said signaltransfer means including a first summing circuit connected to add a predetermined first portion of the Lf signal from the first input terminal to a like first portion of the Lb signal from the second input terminal to produce a first sum signal, a second summing circuit connected to add said predetermined first portion of the Rf signal from the fourth input terminal to a like portion of the Rb signal from the third input terminal to produce a second sum signal, a third summing circuit connected to combine in phase opposition a second predetermined portion smaller than said first predetermined portion of the Lf signal from the first input terminal and a like smaller portion of the Lb signal from the second input terminal to produce a third sum signal, a fourth summing circuit connected to combine in phase opposition said second predetermined smaller portion than said first predetermined portion of the Rf signal from the fourth input terminal and a like smaller portion of the Rb signal from the third input terminal to produce a fourth sum signal, a fifth summing circuit connected to add said first sum signal to said fourth sum signal and first all-pass phase-shifting means operative to cause the first sum signal received by said fifth summing means to be substantially in phase quadrature with the fourth sum signal received by said fifth summing circuit, a sixth summing circuit connected to add said second sum signal to said third sum signal and second all-pass phase-shifting means operative to cause the second sum signal received by said sixth summing circuit to be substantially in phase quadrature with the third sum signal received by said sixth summing circuit, and means connecting the output terminals of said fifth and sixth summing circuits to said first and second output terminals, respectively, thereby to produce at said first output terminal a composite signal LT consisting of said predetermined first portion of said Lf and Lb signals and said second predetermined smaller portion of said Rf and Rb signals and wherein the Rf signal is substantially in quadrature with one of the Lf and Lb signals and the Rb signal is substantially in quadrature with the other of the Lf and Lb signals, and to produce at said second output terminal a composite signal RT consisting of said predetermined first portion of said Rf and Rb signals and said second predetermined smaller portion of the Lf and Lb signals and wherein the Lf signal is substantially in quadrature with one of the Rb and Rf signals and the Lb signal is substantially in quadrature with the other of the Rf and Rb signals.
 2. Apparatus according to claim 1, wherein said predetermined first portion is substantially the decimal fraction 0.924 and said predetermined second smaller portion is substantially the decimal fraction 0.383.
 3. Encoding apparatus according to claim 1, wherein for in-phase input signals Lf, Lb, Rb and Rf said summing circuits and said phase-shifting means are operative to cause the Lf and Lb signals in the composite signal LT and the Rf and Rb signals in the composite signal RT to all be substantially in phase, to cause the Rf and Rb signals in the composite signal LT to be substantially in phase opposition with one another, and to cause the Lf and Lb signals in the composite signal RT to be substantially in phase opposition with one another.
 4. Encoding apparatus according to claim 1, wherein the said all-pass phase-shifting means are operative to shift the phase of said first, second, third and fourth sum signals to cause the relative phase angles of said first, second, third and fourth sum signals to be 90*, 135*, 45* and 0*, respectively.
 5. Decoding apparatus for reproducing four different signals from two composite signals LT and RT wherein the LT signal contains a predetermined first portion of signals designated Lf and Lb and a second predetermined smaller portion than than said first predetermined portion of signals designated Rf and Rb and wherein the Rf signal is substantially in quadrature with one of the Lf and Lb signals and the Rb signal is substantially in quadrature with the other of said Lf and Lb signals and wherein the RT signal contains said predetermined first portion of the said Rf and Rb signals and said second predetermined smaller portion of the said Lf and Lb signals and wherein the Lf signal is substantially in quadrature with one of said Rf and Rb signals and the Lb signal is substantially in quadrature with the other of said Rf and Rb signals, said apparatus comprising: first and second input circuits to which the LT and RT signals are respectively applied, said input circuits including all-pass phase-shifting means operative to shift the relative phase of said LT and RT signals by a predetermined angle to position corresponding components thereof either in phase or in phase opposition to permit later addition or subtraction of said corresponding components, and first, second, third and fourth signal-combining networks, each having first and second input terminals and an output terminal, each connected to receive at its first and second input terminals a signal from said first and second input circuits, respectively, said first signal-combining network being operative to combine a portion of the signal from said first input circuit equal to said predetermined first portion with the inverse of a portion of the signal from said second input circuit equal to said predetermined second portion and to produce at its output terminal a first output signal containing one of said Lf and Lb signals as its predominant component, said second signal-combining network being operative to combine a portion of the signal from said first input circuit equal to said predetermined first portion with a portion of the signal from said second input circuit equal to said predetermined second portion and to produce at its output terminal a second output signal containing the other of said Lf and Lb signals as its predominant component, said third signal-combining network being operative to combine a portion of the signal from said first input circuit equal to said predetermined second portion with a portion of the signal from said second input circuit equal to said predetermined first portion and to produce at its output terminal a third output signal containing one of said Rf and Rb signals as its predominant component, and said fourth signal-combining networK being operative to combine the inverse of a portion of the signal from said first input circuit equal to said predetermined second portion with a portion of the signal from said second input circuit equal to said predetermined first portion and to produce at its output terminal a fourth output signal containing the other of said Rf and Rb signals as its predominant component.
 6. Decoding apparatus according to claim 19 wherein said predetermined first portion is substantially the decimal fraction 0.924 and said predetermined second smaller portion is substantially the decimal fraction 0.383.
 7. Decoding apparatus according to claim 5, wherein said input circuits are operative to introduce a relative phase shift of substantially 90* between said LT and RT composite signals, and wherein said first, second, third and fourth output signals respectively contain the Lf, Lb, Rb and Rf signals as its predominant component.
 8. Decoding apparatus according to claim 5, wherein said input circuits are operative to introduce a relative phase shift of substantially 45* between the LT and RT composite signals, and wherein said first, second, third and fourth output signals respectively contain the Lb, Lf, Rf and Rb signals as its predominant component.
 9. Encoding apparatus for matrixing four input signals designated Lf, Lb, Rb and Rf into two composite signals, said apparatus comprising: first, second, third and fourth input terminals to which said Lf, Lb, Rb and Rf are respectively applied, first and second output terminals, circuit means including means for shifting the phase of said Rf input signal at said fourth input terminal by a reference phase-shift angle and for shifting the phase of said Lb input signal at said second input terminal by said reference phase-shift angle plus 90* and operative to couple first and second predetermined portions, respectively, of said phase shifted Rf and Lb signals to said first output terminal, circuit means including means for shifting the phase of said Rb input signal at said third input terminal by said reference phase-shift angle and for shifting the phase of said Lf input signal at said first input terminal by said reference phase-shift angle plus 90* and operative to couple said first and second predetermined portions, respectively, of said phase-shifted Rb and Lf signals to said first input terminal, circuit means including means for shifting the phase of said Lb input signal at said second input terminal by said reference phase-shift angle and for shifting the phase of said Rf input signal at said fourth input terminal by said reference phase-shift angle plus 90* and operative to couple said first and second predetermined portions, respectively, of said phase-shifted Lb and Rf signals to said second output terminal, and circuit means including means for shifting the phase of said Lf input signal at said first input terminal by said reference phase-shift angle and for shifting the phase of said Rb input signal at said third input terminal by said reference phase-shift angle plus 90* and operative to couple said first and second predetermined portions, respectively, of said phase-shifted Lf and Rb signals to said second output terminal.
 10. Encoding apparatus in accordance with claim 9, wherein said first and second predetermined portions are respectively the decimal fractions 0.383 and 0.924. 